MD-2 (source code)
= MD-2
{c}
<MD-2>, myeloid differentiation factor 2, is a <protein> that associates with <Toll-like receptor 4> in recognition of bacterial <lipopolysaccharide>. Its ligand-binding pocket and its contacts with the receptor help connect molecular recognition to assembly of a signaling complex. It is distinct from <lipopolysaccharide-binding protein> and <CD14>, other participants in recognition of the same microbial material.<The structural basis of lipopolysaccharide recognition by the TLR4–MD-2 complex by Beom Seok Park and others>[Abstract: receptor heterodimer, ligand pocket and assembly]{ref}
A 2009 <crystal structure> revealed two copies of the receptor–MD-2–ligand unit in a symmetric assembly. Five of the examined ligand's six lipid chains occupied the <MD-2> pocket; the remaining chain contributed to contact with the neighboring receptor. The location of a chemical group, rather than just the number of ligand molecules present, helped explain the assembly interface.<The structural basis of lipopolysaccharide recognition by the TLR4–MD-2 complex by Beom Seok Park and others>[Primary Abstract: five buried chains, exposed sixth chain and receptor multimer]{ref}
For an original counting example, two such ligand-bound units contain two <MD-2> molecules, two <Toll-like receptor 4> molecules and two ligand molecules. Six molecules in total do not imply six independent signaling units: their organization into a particular complex is part of the mechanism. The arithmetic counts the modeled assembly; it is not a dose–response law.
A useful experimental comparison would measure ligand binding and a downstream response separately while perturbing the interface. Preserved binding with impaired response would distinguish recognition from productive assembly. This is an experiment-design illustration, not a reported result from the structural study. A <crystal structure> supplies spatial evidence, while a cellular response assay tests a different level of function.
Back to article page
